This invention relates generally to the field of spread spectrum communication in Code Division Multiple Access (CDMA) wireless communication systems and more particularly to apparatus and methods for receiving time misalignment resistant spread spectrum signals.
Direct sequence modulation of a carrier signal is a known technique for generating wide-band, low power density signals which have statistical properties similar to random noise. In a common method of direct-sequence spread spectrum modulation, a pseudo-random chip sequence (also called a pseudo-noise code sequence or a PN code sequence) is used to encode data which is then placed on a carrier waveform. The chip rate of the pseudo-random sequence is usually much higher than the data rate. The resulting encoded signal is generally spread across a bandwidth exceeding the bandwidth necessary to transmit the data, hence the term xe2x80x9cspread spectrum.xe2x80x9d
At the receiving end, the receiver recovers the desired signal by correlating the received spread spectrum signal with the PN sequence (spreading sequence) corresponding to the desired signal when it is able to match this sequence to a sufficient degree. To do so, the receiver generates the same PN chip sequence locally, synchronizes its chip sequence to the received chip sequence, and tracks the signal by maintaining synchronization during reception of data.
For severely multipath dispersive environments, the downlink capacity of CDMA systems, such as IS-95, is reduced by intra-cell interference. Downlink signals in a cell are orthogonal or substantially orthogonal at the transmitter (hereinafter the term orthogonal or substantially orthogonal includes the terms orthogonal and/or substantially orthogonal), but tend to suffer a loss of orthogonality due to multipath propagation, time jitter (imprecise implementation leading to errors), etc.
The uplink capacity of CDMA systems, such as IS-95, is reduced because the uplink signals are not orthogonal when they are transmitted. However, orthogonal signals may be used for the uplink if different user symbols are received with timing alignment. For nondispersive channels, time alignment of the signals results in orthogonality and rejection of the intra-cell interference. In reality, however., wireless channels are dispersive and propagation along multiple paths causes time misalignment in which case multipath components will interfere with other multipath components having different delays. In this case it may only be possible to align a single component from each signal. Therefore, orthogonality is partially lost due to the interference between non-aligned terms.
Systems are currently being developed for transmitting signals which are resistant to time misalignment caused by multipath delays. While current receivers should be able to receive these signals, they will do so inefficiently.
Accordingly there exists a need for a system and method of efficiently receiving a code which is resistant to time misalignment (delay) caused by multipath.
There also exists a need for a system and method of receiving a code which is resistant to interference caused by time jitter of the system.
A need also exists for a system and method of receiving a code which is resistant to interference caused by time misalignment of the signals received from different users.
Accordingly it is an object of the present invention to provide systems and methods of receiving codes in CDMA systems which are resistant to effects of time misalignment caused by multipath propagation delays.
It is another object of the present invention to provide systems and methods of receiving codes which are resistant to interference caused by time jitter of the system.
It is another object of the present invention to provide systems and methods of receiving codes which are resistant to effects of time misalignment of signals from different users.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.
In accordance with the teachings of the present invention, these and other objects may be accomplished by the present invention, which receives and processes signals modulated with a carrier frequency, wherein the signals include spreading chip sequences having at least one zero and at least one non-zero.
An embodiment of the invention includes a method of receiving and despreading spread spectrum signals wherein the spread spectrum signals include spreading chip sequences having at least one zero and at least one non-zero chip. The embodiment includes receiving a spread spectrum communication signal, sampling the spread spectrum communication signal, generating a despreading code, synchronizing the despreading code with the sampled spread spectrum communication signal, and despreading the sampled spread spectrum communication signal with the synchronized despreading code. The despreading is performed on a subset of the chips by omitting arithmetic operations involving at least one zero chip.
Another embodiment of the invention includes a method of receiving signals modulated with a carrier frequency, wherein the signals include spreading chip sequences having at least one zero chip. This embodiment includes receiving a modulated spread spectrum communication signal at an antenna separating the carrier frequency and converting the signal to baseband. The baseband signal is then sampled. A despreading code is generated and synchronized with the baseband signal. The baseband signal is correlated with the despreading code such that arithmetic operations involving at least one zero chip are omitted during the correlation process.
Still another embodiment of the invention includes a receiver for receiving spread spectrum communication signals modulated with a carrier frequency, wherein the signals include spreading chip sequences having at least one zero. The receiver includes receiver means for receiving a modulated spread spectrum communication signal, code generator means coupled to the receiver means for generating a despreading code within the receiver. It includes RF processing means for separating the carrier frequency from the modulated spread spectrum communication signal. It includes synchronizer means for synchronizing the separated spread spectrum communication signal with the despreading code and, despreading means for despreading the spread spectrum communication signal with the despreading code. The despreading is performed on a subset of the chips by omitting arithmetic operations involving at least one zero chip.
In yet another embodiment, the invention includes a receiver configured to receive signals modulated with a carrier frequency, wherein the signals include spreading chip sequences having at least one zero. The receiver includes an antenna capable of receiving a spread spectrum communication signal, a despreading code generator connected to the antenna, and a demodulator connected to the antenna and configured to separate the carrier frequency from the spread spectrum communication signal. It includes a searcher connected to the demodulator and configured to synchronize the spread spectrum communication signal with a despreading code generated by the despreading code generator. The searcher omits arithmetic operations involving at least one zero chip. It also includes a Rake receiver connected to the demodulator, configured to despread the separated spread spectrum communication signal by omitting arithmetic operations involving at least one zero chip.
The invention will next be described in connection with certain exemplary embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.